FM modulation system using quadrature modulator

ABSTRACT

The invention provides a FM modulation system using a quadrature modulator, wherein a circuit for exclusive use in FM modulation and a change-over means can be dispensed with. In this system, modulating signals are inputted to a baseband signal processing circuit, converted into digital signals after integration, and turned into an analog signal by reading out a sine wave signal pre-stored in a sin ROM, thereby generating a baseband signal I for an equi-phase signal and a baseband signal Q for a quadrature signal. By adding together results of multiplication of the baseband signal I for the equi-phase signal and the baseband signal Q for the quadrature signal by a local signal of cosine and a sine wave signal obtained by phase shifting the local signal through 90 degrees, respectively, a quadrature modulation signal containing a FM modulated signal and the maximum frequency deviation can be generated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a FM modulation system using a quadraturemodulator, and particularly, to a FM modulation system using aquadrature modulator, wherein a FM modulator and a system for taking outan output signal therefrom can be dispensed with through processing of abaseband signal and subsequent operation of quadrature modulationthereof in coping with a dual mode communications system incorporating aFM (frequency modulation) method in combination with CDMA (code divisionmultiple access) techniques.

2. Description of the Related Art

The CDMA techniques used by conventional communications servers in Japanalways incorporate both analog TACS (total access communication system)and a CDMA method, and since respective modulation methods differ fromeach other, the TACS adopts a FM modulation while the CDMA method adoptsquadrature modulation.

FIG. 4 is a block diagram showing the construction of a conventionaldual mode communications system incorporating both the TACS and CDMAmethod.

As shown in FIG. 4, in the TACS adopting frequency modulation, amodulating signal fm is inputted to a FM modulator 1 for frequencymodulation, and subsequently, after converting frequency throughmultiplication thereof by a local signal LA at a multiplier (mixer) 2, amodulated wave is taken out by switching a change-over device 3.

Meanwhile, in the CDMA method, a baseband signal ba and the local signalLA are inputted to a quadrature modulator 4 for quadrature modulation,and a modulated wave is taken out by switching the change-over device3.

Thus, with the conventional dual mode communications systemincorporating both the TACS and CDMA method, the TACS employs the FMmodulator 1 wherein an output signal from an oscillator (not shown) isfrequency modulated in a frequency band in the range of about 40 to 50MHz by the modulating signal fm such as an audio signal, and furtherconverted to the order of 800 MHz in frequency by the multiplier (mixer)2.

Accordingly, the oscillator is a must for the FM modulator 1, and has afrequency band in the range of about 40 to 50 MHz. Since the outputsignal from the oscillator which is frequency modulated in the frequencyband in the range of about 40 to 50 MHz by the modulating signal fm,always has side bands generated at the sides thereof, and is furtherconverted to the order of 800 MHz in frequency after multiplication bythe local signal LA at the multiplier 2, removal of the side bands iscalled for.

For removal of the side bands, however, a filter having a characteristicof sharp frequency cut will be called for, but it is technically verydifficult and very high in cost to manufacture such a filter having thecharacteristic of sharp frequency cut.

Although it is also conceivable as an alternative to lower theoscillation frequency of the local signal, this will entail necessity ofincreasing the oscillation frequency of the FM modulator, and as it isdifficult to manufacture a FM modulator at high frequency, an increasein cost will result.

SUMMARY OF THE INVENTION

The invention has been developed to solve aforesaid problems encounteredwith conventional systems, and an object of the invention is to providea FM modulation system using a quadrature modulator, capable of takingout a FM modulated signal from a quadrature modulator so that aconventional FM modulator and a system for taking out the FM modulatedsignal can be dispensed with, attaining simplification in a circuitscale and reduction in cost.

To this end, the FM modulation system using the quadrature modulatoraccording to the invention comprises a baseband processing circuit forgenerating a baseband signal for a quadrature signal and a basebandsignal for an equi-phase signal by reading out a pre-stored sine wavesignal of a predetermined frequency and converting the same into ananalog signal with the use of an address signal obtained by digitizing amodulating signal input through a predetermined sampling process, andthe quadrature modulator for generating a FM modulated high frequencysignal by adding together a signal obtained through multiplication ofthe baseband signal for the equi-phase signal by a local signal of acosine signal at respective predetermined frequencies, and a signalobtained through multiplication of a sine signal resulting fromphase-shifting the local signal through 90° by the baseband signal forthe quadrature signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block schematic diagram showing the construction of a firstembodiment of a FM modulation system using a quadrature modulatoraccording to the invention.

FIG. 2 is a block schematic diagram showing the internal structure of abaseband signal processing circuit of the FM modulation system using thequadrature modulator shown in FIG. 1.

FIG. 3 is a block schematic diagram showing the internal structure ofthe quadrature modulator of the FM modulation system using thequadrature modulator shown in FIG. 1.

FIG. 4 is a block schematic diagram showing the construction of aconventional dual mode communications system incorporating TACS incombination with a CDMA method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a FM modulation system using a quadrature modulatoraccording to the invention is described hereinafter with reference tothe accompanying drawings. FIG. 1 is a block diagram showing theconstruction of a first embodiment of the invention.

In FIG. 1, the FM modulation system according to the first embodimentcomprises a baseband signal processing circuit 11 for receiving amodulating signal V (t) for executing a predetermined processing,thereby outputting a baseband signal I for a quadrature modulatedequi-phase signal and baseband signal Q for a quadrature signal, and aquadrature modulator 12 for receiving the baseband signal I for theequi-phase signal and the baseband signal Q for the quadrature signalwhich are multiplied with a local signal fc of a cosine (abbreviated"cos") wave signal and a sine (abbreviated "sin") wave signal producedby phase-shifting the local signal fc through 90°, respectively, therebyoutputting the quadrature modulated equi-phase signal and the quadraturesignal.

FIG. 2 is a block diagram showing the internal structure of the basebandsignal processing circuit 11 of the FM modulation system according tothe first embodiment of the invention.

In FIG. 2, by inputting modulating signals V (t)=VO cos 2π fm t to anintegrating circuit 11a, the modulating signals V (t) are integrated,and an integrated output signal Vc (t) at the output terminal of theintegrating circuit 11a is represented by the following formula (1);

    Vc(t)=VO /√{1+(2π fm CR)2}·cos (2π fm CRt+ψ-π/2)                                         (1)

where ψ=tan-1 (1/2π fm CR) and C represents capacitance and R representsresistance.

The modulating signals V (t) are integrated in the integrating circuit11a because a signal outputted from a sin ROM 11c as memory meansdescribed later is sin {VO cos (2π fm t)} where fm denotes a modulationfrequency.

The integrated output signal Vc (t) produced by integrating themodulating signals V (t) in the integrating circuit 11a is delivered toan analog-digital (referred to hereinafter as A/D) converter 11b.

If with respect to Vc (t) in the formula (1) above, 2π fm CR is largeenough to be able to ignore "1", (t) can be represented by the followingequation;

    (t)=(VO/2π fm CR)·sin 2π fm=(VO/2π fm CR)·sin (2π fm t)=[{(1/2π CR)·VO}/fm]·sin (2π fm t)(2)

The integrated output signal Vc (t) is sent out to the A/D converter 11bwhere the integrated output signal Vc (t) is converted into a digitalsignal through sampling with a sampling signal fs.

An output signal from the A/D converter 11b, that is, the digital signalconverted as described is inputted to a sin ROM 11c as an address signalfor reading out data written in the sin ROM 11c as the memory means.

To obtain the address signal, A/D conversion data D of the A/D converter11b is found in the following manner.

The A/D conversion data D of the A/D converter 11b at time t is found bythe following formula (3);

    {Vc(t)/VREF}=D/(2n -1)                                     (3)

where VREF is the full scale of the A/D converter 11b, and n is thenumber of bits.

From the formula (3), the following is obtained;

    D={Vc(t)/VREF}·(2n -1)                            (4)

Using the A/D conversion data D according to the formula (4) as addressdata in the sin ROM 11c, and assuming the full scale VREF of the A/Dconverter 11b as (2n -1)=2π, sin data written in the sin ROM 11c is readout, and converted into an analog signal by a digital-analog converter(referred to hereinafter as D/A converter 11d), thereby finding thebaseband signal I for the quadrature modulated equi-phase signal by thefollowing formula (5);

    I=cos {(mf·V/fm)·sin (2π fm·t)}(5)

Then, the quadrature modulated baseband signal Q for the quadraturesignal is found by the following formula (6);

    Q=sin {(mf·V/fm)·sin (2π fm·t)}(6)

The baseband signal I for the equi-phase signal and the baseband signalQ for the quadrature signal, thus produced, are outputted from the D/Aconverter 11d in FIG. 2, and inputted to the quadrature modulator 12,for quadrature modulation after multiplication by the local signal fc,thereby producing a quadrature modulated signal.

FIG. 3 shows the internal structure of the quadrature modulator 12. InFIG. 3, the baseband signal I for the equi-phase signal (for brevity,expressed as I=A cos φ i in FIG. 3) and the baseband signal Q for thequadrature signal (also for brevity, expressed as Q=A sin φ i in FIG. 3)are inputted to a first input terminal of first and second multipliers(mixers) 12a, and 12b, respectively, and the local signal fc {cos (2π fct)} is inputted to the second input terminal of the first multiplier12a.

The local signal fc is also inputted to a 90° phase shifter 12c. Thelocal signal fc with the phase thereof shifted by 90°, after inputted tothe 90° phase shifter 12c, is inputted as -sin (2π fc t) to the secondinput terminal of the second multiplier 12b.

The first multiplier 12a executes multiplication of the local signal fc{cos (2π fc t)} by the baseband signal I for the equi-phase signal, andoutputs a multiplication result to the first input terminal of an adder12d.

The second multiplier 12b execute multiplication of the baseband signalQ for the quadrature signal by the local signal fc whose phase isshifted by 90°, {-sin (2π fc t)}, and outputs a multiplication result tothe second input terminal of the adder 12d.

The adder 12d adds together output signals from the respectivemultipliers 12a, 12b, and outputs the quadrature modulated signal Emodas expressed by the following formula (7);

    Emod=A cos (2π fc t+φi)=A cos φi·cos 2π fc t-A sin φ i·cos 2π fc t                           (7)

A cos φ i according to the formula (7) represents the baseband signal Ifor the equi-phase signal inputted to the quadrature modulator 12, and Asin φ i represents the baseband signal Q for the quadrature signal.

In the FM modulation system using the quadrature modulator according tothe invention, a FM modulated signal must be included in the quadraturemodulated signal Emod in order to dispense with a conventional FMmodulator.

Accordingly, the quadrature modulated signal Emod is comparedhereinafter with a phase modulating signal EPM and a frequencymodulating signal EFM, respectively, on the basis of a modulating waveE=cos φ (t) where φ (t)=instantaneous phase, assuming:

an instantaneous phase of the quadrature modulated signal Emod

    φ(t)=2π fc t+φi                                 (8),

an instantaneous phase of the phase modulating signal EPM

    φ(t)=2π fc t+2π·mPV(t)                  (9),

an instantaneous phase of the frequency modulating signal EFM

    φ(t)=2π fe t+2π·mf ∫V(t)dt         (10),

and

a phase angle in the formula (8)φ i=-π˜+π. Also, mP in the formula (9)denotes a coefficient, and V (t) a modulating signal. Further, mf in theformula (10) denotes a coefficient, and 2π is employed to find a phaseby the radian.

Now, if a modulating frequency f=fm, and the modulating signal V (t)=Vcos (2π fm t), the following formula (11) results;

    the phase modulating signal EPM=cos {2π fc t+2π·mP V cos (2π fm t)}                                             (11)

By employing the modulation index β=2π·mP V, the formula (11) can beexpressed as follows;

    EPM=cos {2π fc t+β cos 2π fm t)}                (12).

Then, on the basis of ∫V (t)dt=(V/2π fm) sin (2π fm t), the frequencymodulating signal EFM can be expressed as follows;

    EFM=cos {2π fc t+2π mf(V/2π fm) sin (2π fm t)}=cos {2π fc t+(mf V/fm) sin (2π fm t)}                             (13).

The formula (13) above indicates that the instantaneous phase φ (t) ofthe FM modulating wave can be represented by (mf V/fm) sin (2π fm t).

And, an instantaneous frequency f of the FM modulating wave can berepresented by the following formula;

    (d/dt)φ(t)=2π{fc+mf V cos (2π fm t)}             (14).

In the formula (14) above, mf V represents the maximum frequencydeviation Δf, and the formula (14) is replaced by the following formula(15), employing the maximum frequency deviation Δf;

    EFM=cos {2π fc t+(Δf/fm)·sin (2π fm t)}(15).

In comparing the FM modulating wave in the formula (15) with a phasemodulating wave in the formula (12), the following relation exists amongthe modulation index β in the formula (12), maximum frequency deviationΔf and modulating frequency fm in the formula (15);

    β=Δf/fm.

As is evident from the foregoing, factors inside braces in the formula(12) above correspond to the instantaneous phase φ (t) of the phasemodulating wave, and factors inside braces in the formula (15) abovecorrespond to the instantaneous phase φ (t) of the frequency modulatingwave.

Accordingly, the quadrature modulated signal Emod=A cos (2π fc t +φi)produced by inputting the baseband signal I for the equi-phase signalI=A cos φi and the baseband signal Q for the quadrature signal Q=A sinφi to the quadrature modulator 12 for quadrature modulation isessentially phase modulated, and φi in the formula is a quadraturemodulated signal.

For frequency modulation, as EFM=cos {2π fc t+Σφi dt}, φi in the formulais replaced by the factors within braces in the formula (15) above asfollows;

    φi=(Δf/fm)·sin (2π fm t)             (16).

Thus, a FM wave as desired can be produced by the quadrature modulatedsignal Emod=A cos (2π fc t+φi).

By substituting the formula (16) above for φi in the baseband signal Ifor the equi-phase signal=A cos φi, and the baseband signal Q for thequadrature signal=A sin φi, respectively, the following formulae result;

    I=A cos {(Δf/fm)·sin (2π fm t)}          (17)

    Q=A sin {(Δf/fm)·sin (2π fm t)}          (18)

When the baseband signal I for the equi-phase signal=A cos φi, and thebaseband signal Q for the quadrature signal=A sin φi, according to theformula (17) and the formula (18), respectively, are inputted to thequadrature modulator 12 for quadrature modulation, a FM wave of themodulation frequency fm and the maximum frequency deviation Δf isoutputted from the quadrature modulator 12.

Consequently, in a system incorporating the FM process combined with thequadrature modulation process such as the CDMA method, a circuit forexclusive use for frequency modulation can be dispensed with.

In the first embodiment of the invention described hereinbefore, thebaseband signal processing circuit 11 may not be limited to an exampleshown in FIG. 2, but in place of the A/D converter 11a shown in FIG. 2,a clock counter may be used so that an address signal is outputted tothe sin ROM according to the number of counts by a clock signal, dataread out from the sin ROM is converted to analog by the D/A converter11d, and subsequently, integrated in an integrating circuit, producingthe baseband signal I for the equi-phase signal and the baseband signalQ for the quadrature signal.

In the description given above on the construction of the basebandsignal processing circuit 11, mention is made of a hardware aspect only,however, the baseband signal I for the equi-phase signal and thebaseband signal Q for the quadrature signal may be calculated by asoftware instead.

As described, with the FM modulation system using the quadraturemodulator according to the invention, a modulating signal is inputted tothe baseband signal processing circuit so that data of a sin table,pre-stored in the memory means, is read out by the digitized addresssignal, and converted to analog, producing the baseband signal I for theequi-phase signal and the baseband signal Q for the quadrature signalfor quadrature modulation by the quadrature modulator. As a result, thequadrature modulated signal generated by the quadrature modulator cancontain a frequency modulated signal.

Hence, in a system with the FM process and quadrature modulation processsuch as the CDMA method in mixed presence, the circuit for exclusive usefor frequency modulation can be dispensed with, and such components asan oscillator required in a FM modulator, a filter technically difficultto manufacture, and a change-over device for switching between the FMand quadrature modulation processes will no longer be required,contributing to reduction in cost.

What is claimed is:
 1. An FM modulation system comprising:a basebandsignal processing circuit for generating a baseband signal for aquadrature signal and a baseband signal for an equi-phase signal byreading out a sine wave signal of a predetermined frequency pre-storedand converting the same into an analog signal with the use of an addresssignal obtained by digitizing modulating signals input through apredetermined sampling process; and a quadrature modulator forgenerating a FM modulated high frequency signal by adding together asignal obtained through multiplication of the baseband signal for theequi-phase signal by a local signal of a cosine signal at respectivepredetermined frequencies, and a signal obtained through multiplicationof a sine signal resulting from phase-shifting the local signal through90 degrees by the baseband signal for the quadrature signal; and saidbaseband signal processing circuit including:(a) memory means forpre-storing the sine wave signal of the predetermined frequency, (b) acounter for generating the address signal which reads out the sine wavesignal stored in the memory means by use of a clock signal, (c) adigital-to-analog converter for converting data from the memory meanscorresponding to the sine wave signal into an analog signal, and (d) anintegrator for generating the baseband signal for the equi-phase signaland the baseband signal for the quadrature signal by integrating outputsignals of the digital-to-analog converter.
 2. An FM modulation systemcomprising:a baseband signal processing circuit for generating abaseband signal for a quadrature signal and a baseband signal for anequi-phase signal by reading out a sine wave signal of a predeterminedfrequency pre-stored and converting the same into an analog signal withthe use of an address signal obtained by digitizing modulating signalsinput through a predetermined sampling process; a quadrature modulatorfor generating a FM modulated high frequency signal by adding together asignal obtained through multiplication of the baseband signal for theequi-phase signal by a local signal of a cosine signal at respectivepredetermined frequencies, and a signal obtained through multiplicationof a sine signal resulting from phase-shifting the local signal through90 degrees by the baseband signal for the quadrature signal; and saidbaseband signal processing circuit including:(a) an integrator forintegrating the modulating signals, (b) memory means for pre-storing thesine wave signal of the predetermined frequency, (c) ananalog-to-digital converter for converting an output signal of theintegrator into a digital signal by use of a sampling signal at apredetermined frequency and for reading out the sine wave signal storedin the memory means, and (d) a digital-to-analog converter forconverting the sine wave signal read out from the memory means into ananalog signal, and outputting the baseband signal for the quadraturesignal and the baseband signal for the equi-phase signal.